SuDS made simple – Design
A common – but not the only – way of designing SuDS systems is through the consideration of a number of ‘design storms’. The design objective is different for each storm frequency. For example, small storms should be fully infiltrated on site whilst very large storms should be managed in such a way that they don’t do damage (D3).
Figure 1 gives a conceptual SuDS design framework. It is a plot of peak flow rate versus the storm recurrence interval (the reciprocal of frequency of exceedance). Two curves are indicated: the first is labelled ‘Pre-development’ and refers to an idealised relation between peak flow rate and storm recurrence interval for a theoretical catchment that is still in its natural state. As can be seen, no flow is anticipated for low-recurrence interval i.e. very small storms. Precipitation is totally absorbed through a combination of interception storage and infiltration. A point, however, is reached where the rainfall becomes sufficient – a combination of intensity and duration – to result in runoff. Larger storms will generally results in larger peak runoffs trending to theoretical maximum defined by the Regional Maximum Flood (RMF) or Probably Maximum Flood (PMF). The second curve is labelled ‘Post development’ and refers to the situation currently associated with most development. Hardening of the surface and improving the conveyance of drainage channels results in flow for smaller storms that in the case of the pre-development situation, and higher flows for all larger storms. It is only in the case of extreme events that the difference between peak flow rates emanating from the pre-development and post-development situations narrows to eventually disappear as the surface becomes completely saturated – thereby reducing intervention storage and infiltration to zero – and the flow paths become very similar. One of the objectives of the SuDS approach is to bring the peak flow rates (and associated volumes) back to the pre-development situation. It simultaneously attempts to meet the objectives of water quality, providing for amenity and preserving bio-diversity. This makes it inherently more complex than conventional design which typically has only two primary objectives: minimising inconvenience through the minor – usually piped – system, and minimising damage to property and potential loss of life through the major – usually overland flow – system.
Figure 1: Conceptual SuDS design framework
Figure 1 indicates five distinct design ‘zones’ that need to be considered for SuDS design:
i) All precipitation is absorbed through interception storage and infiltration in Zone 1.
ii) As the storm intensity increases, the focus (in Zone 2) moves to the management of runoff quality and quantity.
iii) It is often very difficult to handle water quality issues for all but small storms; past a certain threshold the emphasis starts to move to one of channel bed protection (Zone 3).
iv) There is still need to minimise inconvenience so SuDS must give the equivalent peak overland flow protection offered by conventional systems beyond that achieved in Zones 1-3. This is covered in Zone 4.
v) SuDS need to be designed for major events just like conventional systems. Zone 5 may thus be divided into two: Zone 5a where peak flows may be reduced to pre-development, and Zone 5b where the emphasis moves to minimising damage to property and potential loss of life (D3 = “Don’t Do Damage”!)
Overall, Zones 1-4 cover the equivalent of ‘minor’ system design whilst Zone 5 covers ‘major’ system design. In Zones 1 and 2 the focus is on the improvement of water quality through volume-based sizing of SuDS options. Zones 3-5 focus on flow control. Table 1 defines some key terms that appear in Figure 1. Table 2 these are linked up with the different zones and types of management. Additional explanation follows.
Table 1: Key design terms
Table 2: Links between zones, design objectives and associated treatment / management